Fossil fuels, biomass, waste pyrolysis/gasification gas, biogas, and the like have methane (CH4) and carbon dioxide (CO2) for their main ingredients, and these two gases are greenhouse gases known as main causative materials of climate change. The reforming technology for converting these greenhouse gases into a high-quality fuel has received attention and has been studied continuously. Methane steam reforming (CH4+H2O↔CO+3H2) has already been applied in the industry. However, recently, methane dry reforming (CH4+CO2↔2CO+2H2) has been recognized as a more attractive method due to reduction of greenhouse gas and effective chemical conversion energy.
In order to convert methane and carbon dioxide into hydrogen or carbon monoxide, high-temperature reaction conditions need to be satisfied or an appropriate help of catalyst is needed. In order to overcome this problem, a new catalyst excellent in reforming conversion reaction without carbon deposition has recently been developed. A catalyst formed of a noble metal or transition metal having a high catalytic ability has been widely used. However, a noble metal catalyst is too expensive and a transition metal catalyst is subject to deterioration in catalytic ability due to catalytic deactivation caused by carbon deposition.
In recent years, basic studies on CH4—CO2 reforming using various kinds of carbon materials are being conducted. Such carbon materials are typically used as a catalyst or catalyst carrier, and activated carbon, coal char, semi-coke and bio-char are mainly used. These studies have been widely conducted about a carbon material itself, the catalytic activity of modified carbon-containing catalyst, and reforming reaction characteristics for each different operating condition. A carbon material-based catalyst is cheaper than conventional catalysts and has high catalytic activity and rarely has the sulfur poisoning problem.
Bio-char which is a carbide produced by pyrolysis of biomass can be used as a source for combustion, gasification or activated carbon. There has been a lot of attention on the production of a synthetic gas by gasification of bio-char. If bio-char is used in methane reforming, gasification of bio-char is an essential process. Therefore, a study on carbon gasification in a reforming reaction is very significant. However, there is barely any in-depth technology on this field.
A microwave heating method is excellent in energy efficiency as compared with a conventional hot-air or electrical heating method and has thermal characteristics of being excellent in rapid heating, selective heating, and homogenous heating. So far, the microwave heating method has been widely applied to reduction of environmental pollutants, pyrolysis/gasification of biomass, and drying of materials.
Recently, the microwave heating method has been applied to the above-described methane dry reforming of a carbon-based catalyst. Such studies confirmed that a carbon material is an excellent microwave receptor and has a higher gas reforming conversion rate than the conventional heating methods. Further, it was reported that the selectivity of a product gas is improved and the carbon deposition is reduced. However, technology about a reforming apparatus using carbide as a microwave receptor has not been developed.
The conventional heating methods are affected by conduction and convection of a material and have slow heat transfer and low heating efficiency since an object is heated from the surface to the inside. On the other hand, dielectric heating such as microwave heating converts electromagnetic energy into heat energy, and, thus, it is a kind of energy conversion rather than heating. That is, as illustrated in FIG. 3, microwaves pass through a target object and then are stored as energy in the target object and converted into heat therein, and, thus, heating is conducted and a temperature at the center is higher than that on the surface.
There have been known three mechanisms for enhancing chemical reaction when a microwave (MW) irradiation technique. The first is a thermal effect by which high-temperature reaction of a polar material irradiated with microwaves is increased, the second is a specific MW effect by which reaction activity is improved by microwave internal transfer core volumetric heating unlike conventional simple surface heating as described above, and the third is a non-thermal effect which is a chemical transformation acceleration effect on vibration of chemical species when microwaves are irradiated.
Technology of converting a biogas or product gas into a high-grade reforming gas can be roughly classified into steam reforming, CO2 reforming, oxygen reforming, and autothermal reforming in terms of reaction as illustrated in FIG. 4.
The steam reforming refers to a method of reforming by injecting overheated steam into a reforming apparatus. In this case, the reforming apparatus is large and a reaction rate is relatively low. However, due to its advantages such as the amount of gas processed and a high hydrogen production yield rate, the steam reforming is most commonly used.
The CO2 reforming is getting attention in terms of the use of energy conversion of greenhouse gas. In terms of economics of preparing a synthetic gas, CO2 reforming of methane has been evaluated as being equivalent to steam reforming of methane.
The oxygen reforming refers to reforming via incomplete combustion of a fuel and its whole reaction is operated as an exothermic reaction. The amounts of air and oxygen are determined by reactive equivalents, and a partial oxidation process is a high-temperature exothermic reaction and maintains a temperature of a reactor to a very high level. This method does not require an indirect heat transfer apparatus and the whole process is simpler. Further, the oxygen reforming is rapid in exothermic reaction and initial startup and excellent in load response.
The autothermal reforming is a complex reaction of steam reforming and partial oxidation reaction and an appropriate mix of the advantage of maintaining an appropriate exothermic reaction in the partial oxidation reaction and the advantage of a great amount of hydrogen production in the steam reforming. Particularly, it is easy to select a material of a reactor and carbon deposition less occurs within a system as compared with the other reforming reactions.
As described above, the existing reforming processes have their own characteristics. The steam reforming requires high temperature (700 to 800° C.) and pressure (3 to 25 atm) and uses a catalyst to increase a hydrogen conversion rate, and this can be polluted by sulfur or nitrogen and shortened in lifetime. The CO2 reforming needs to maintain a predetermined operating pressure (1 to 10 atm) of a reactor and requires an external heating source to maintain a temperature to about 400 to 1,000° C. due to high endothermic reaction (247 kJ/mol). Further, according to the oxygen reforming, it is not easy to obtain high-purity hydrogen due to nitrogen components in air used for combustion. Furthermore, in the autothermal reforming, when steam and air react with methane, it is difficult to control an internal temperature of a reforming apparatus.
A microwave reforming apparatus of the present disclosure uses a carbon receptor and thus uses its own internal reaction heat at the time of reforming. Therefore, it is excellent in time required for normal operation and efficiency.